1 What is the Asset Pipeline?

The asset pipeline provides a framework to concatenate and minify or compress
JavaScript and CSS assets. It also adds the ability to write these assets in
other languages and pre-processors such as CoffeeScript, Sass and ERB.

The asset pipeline is technically no longer a core feature of Rails 4, it has
been extracted out of the framework into the
sprockets-rails gem.

The asset pipeline is enabled by default.

You can disable the asset pipeline while creating a new application by
passing the --skip-sprockets option.

rails new appname --skip-sprockets

Rails 4 automatically adds the sass-rails, coffee-rails and uglifier
gems to your Gemfile, which are used by Sprockets for asset compression:

gem 'sass-rails'
gem 'uglifier'
gem 'coffee-rails'

Using the --skip-sprockets option will prevent Rails 4 from adding
sass-rails and uglifier to Gemfile, so if you later want to enable
the asset pipeline you will have to add those gems to your Gemfile. Also,
creating an application with the --skip-sprockets option will generate
a slightly different config/application.rb file, with a require statement
for the sprockets railtie that is commented-out. You will have to remove
the comment operator on that line to later enable the asset pipeline:

# require "sprockets/railtie"

To set asset compression methods, set the appropriate configuration options
in production.rb - config.assets.css_compressor for your CSS and
config.assets.js_compressor for your JavaScript:

The sass-rails gem is automatically used for CSS compression if included
in Gemfile and no config.assets.css_compressor option is set.

1.1 Main Features

The first feature of the pipeline is to concatenate assets, which can reduce the
number of requests that a browser makes to render a web page. Web browsers are
limited in the number of requests that they can make in parallel, so fewer
requests can mean faster loading for your application.

Sprockets concatenates all JavaScript files into one master .js file and all
CSS files into one master .css file. As you'll learn later in this guide, you
can customize this strategy to group files any way you like. In production,
Rails inserts an MD5 fingerprint into each filename so that the file is cached
by the web browser. You can invalidate the cache by altering this fingerprint,
which happens automatically whenever you change the file contents.

The second feature of the asset pipeline is asset minification or compression.
For CSS files, this is done by removing whitespace and comments. For JavaScript,
more complex processes can be applied. You can choose from a set of built in
options or specify your own.

The third feature of the asset pipeline is it allows coding assets via a
higher-level language, with precompilation down to the actual assets. Supported
languages include Sass for CSS, CoffeeScript for JavaScript, and ERB for both by
default.

1.2 What is Fingerprinting and Why Should I Care?

Fingerprinting is a technique that makes the name of a file dependent on the
contents of the file. When the file contents change, the filename is also
changed. For content that is static or infrequently changed, this provides an
easy way to tell whether two versions of a file are identical, even across
different servers or deployment dates.

When a filename is unique and based on its content, HTTP headers can be set to
encourage caches everywhere (whether at CDNs, at ISPs, in networking equipment,
or in web browsers) to keep their own copy of the content. When the content is
updated, the fingerprint will change. This will cause the remote clients to
request a new copy of the content. This is generally known as cache busting.

The technique sprockets uses for fingerprinting is to insert a hash of the
content into the name, usually at the end. For example a CSS file global.css

global-908e25f4bf641868d8683022a5b62f54.css

This is the strategy adopted by the Rails asset pipeline.

Rails' old strategy was to append a date-based query string to every asset linked
with a built-in helper. In the source the generated code looked like this:

/stylesheets/global.css?1309495796

The query string strategy has several disadvantages:

Not all caches will reliably cache content where the filename only differs by
query parameters

Steve Souders recommends,
"...avoiding a querystring for cacheable resources". He found that in this
case 5-20% of requests will not be cached. Query strings in particular do not
work at all with some CDNs for cache invalidation.

The file name can change between nodes in multi-server environments.

The default query string in Rails 2.x is based on the modification time of
the files. When assets are deployed to a cluster, there is no guarantee that the
timestamps will be the same, resulting in different values being used depending
on which server handles the request.

Too much cache invalidation

When static assets are deployed with each new release of code, the mtime
(time of last modification) of all these files changes, forcing all remote
clients to fetch them again, even when the content of those assets has not changed.

Fingerprinting fixes these problems by avoiding query strings, and by ensuring
that filenames are consistent based on their content.

Fingerprinting is enabled by default for both the development and production
environments. You can enable or disable it in your configuration through the
config.assets.digest option.

2 How to Use the Asset Pipeline

In previous versions of Rails, all assets were located in subdirectories of
public such as images, javascripts and stylesheets. With the asset
pipeline, the preferred location for these assets is now the app/assets
directory. Files in this directory are served by the Sprockets middleware.

Assets can still be placed in the public hierarchy. Any assets under public
will be served as static files by the application or web server when
config.serve_static_files is set to true. You should use app/assets for
files that must undergo some pre-processing before they are served.

In production, Rails precompiles these files to public/assets by default. The
precompiled copies are then served as static assets by the web server. The files
in app/assets are never served directly in production.

2.1 Controller Specific Assets

When you generate a scaffold or a controller, Rails also generates a JavaScript
file (or CoffeeScript file if the coffee-rails gem is in the Gemfile) and a
Cascading Style Sheet file (or SCSS file if sass-rails is in the Gemfile)
for that controller. Additionally, when generating a scaffold, Rails generates
the file scaffolds.css (or scaffolds.scss if sass-rails is in the
Gemfile.)

For example, if you generate a ProjectsController, Rails will also add a new
file at app/assets/javascripts/projects.coffee and another at
app/assets/stylesheets/projects.scss. By default these files will be ready
to use by your application immediately using the require_tree directive. See
Manifest Files and Directives for more details
on require_tree.

You can also opt to include controller specific stylesheets and JavaScript files
only in their respective controllers using the following:

When doing this, ensure you are not using the require_tree directive, as that
will result in your assets being included more than once.

When using asset precompilation, you will need to ensure that your
controller assets will be precompiled when loading them on a per page basis. By
default .coffee and .scss files will not be precompiled on their own. See
Precompiling Assets for more information on how
precompiling works.

You must have an ExecJS supported runtime in order to use CoffeeScript.
If you are using Mac OS X or Windows, you have a JavaScript runtime installed in
your operating system. Check ExecJS documentation to know all supported JavaScript runtimes.

You can also disable generation of controller specific asset files by adding the
following to your config/application.rb configuration:

config.generators do |g|
g.assets false
end

2.2 Asset Organization

Pipeline assets can be placed inside an application in one of three locations:
app/assets, lib/assets or vendor/assets.

app/assets is for assets that are owned by the application, such as custom
images, JavaScript files or stylesheets.

lib/assets is for your own libraries' code that doesn't really fit into the
scope of the application or those libraries which are shared across applications.

vendor/assets is for assets that are owned by outside entities, such as
code for JavaScript plugins and CSS frameworks. Keep in mind that third party
code with references to other files also processed by the asset Pipeline (images,
stylesheets, etc.), will need to be rewritten to use helpers like asset_path.

If you are upgrading from Rails 3, please take into account that assets
under lib/assets or vendor/assets are available for inclusion via the
application manifests but no longer part of the precompile array. See
Precompiling Assets for guidance.

2.2.1 Search Paths

When a file is referenced from a manifest or a helper, Sprockets searches the
three default asset locations for it.

The default locations are: the images, javascripts and stylesheets
directories under the app/assets folder, but these subdirectories
are not special - any path under assets/* will be searched.

You can view the search path by inspecting
Rails.application.config.assets.paths in the Rails console.

Besides the standard assets/* paths, additional (fully qualified) paths can be
added to the pipeline in config/application.rb. For example:

config.assets.paths << Rails.root.join("lib", "videoplayer", "flash")

Paths are traversed in the order they occur in the search path. By default,
this means the files in app/assets take precedence, and will mask
corresponding paths in lib and vendor.

It is important to note that files you want to reference outside a manifest must
be added to the precompile array or they will not be available in the production
environment.

2.2.2 Using Index Files

Sprockets uses files named index (with the relevant extensions) for a special
purpose.

For example, if you have a jQuery library with many modules, which is stored in
lib/assets/javascripts/library_name, the file lib/assets/javascripts/library_name/index.js serves as
the manifest for all files in this library. This file could include a list of
all the required files in order, or a simple require_tree directive.

The library as a whole can be accessed in the application manifest like so:

//= require library_name

This simplifies maintenance and keeps things clean by allowing related code to
be grouped before inclusion elsewhere.

2.3 Coding Links to Assets

Sprockets does not add any new methods to access your assets - you still use the
familiar javascript_include_tag and stylesheet_link_tag:

If using the turbolinks gem, which is included by default in Rails 4, then
include the 'data-turbolinks-track' option which causes turbolinks to check if
an asset has been updated and if so loads it into the page:

In regular views you can access images in the public/assets/images directory
like this:

<%= image_tag "rails.png" %>

Provided that the pipeline is enabled within your application (and not disabled
in the current environment context), this file is served by Sprockets. If a file
exists at public/assets/rails.png it is served by the web server.

Alternatively, a request for a file with an MD5 hash such as
public/assets/rails-af27b6a414e6da00003503148be9b409.png is treated the same
way. How these hashes are generated is covered in the In
Production section later on in this guide.

Sprockets will also look through the paths specified in config.assets.paths,
which includes the standard application paths and any paths added by Rails
engines.

Images can also be organized into subdirectories if required, and then can be
accessed by specifying the directory's name in the tag:

<%= image_tag "icons/rails.png" %>

If you're precompiling your assets (see In Production
below), linking to an asset that does not exist will raise an exception in the
calling page. This includes linking to a blank string. As such, be careful using
image_tag and the other helpers with user-supplied data.

2.3.1 CSS and ERB

The asset pipeline automatically evaluates ERB. This means if you add an
erb extension to a CSS asset (for example, application.css.erb), then
helpers like asset_path are available in your CSS rules:

.class { background-image: url(<%= asset_path 'image.png' %>) }

This writes the path to the particular asset being referenced. In this example,
it would make sense to have an image in one of the asset load paths, such as
app/assets/images/image.png, which would be referenced here. If this image is
already available in public/assets as a fingerprinted file, then that path is
referenced.

If you want to use a data URI -
a method of embedding the image data directly into the CSS file - you can use
the asset_data_uri helper.

#logo { background: url(<%= asset_data_uri 'logo.png' %>) }

This inserts a correctly-formatted data URI into the CSS source.

Note that the closing tag cannot be of the style -%>.

2.3.2 CSS and Sass

When using the asset pipeline, paths to assets must be re-written and
sass-rails provides -url and -path helpers (hyphenated in Sass,
underscored in Ruby) for the following asset classes: image, font, video, audio,
JavaScript and stylesheet.

image-url("rails.png") becomes url(/assets/rails.png)

image-path("rails.png") becomes "/assets/rails.png".

The more generic form can also be used:

asset-url("rails.png") becomes url(/assets/rails.png)

asset-path("rails.png") becomes "/assets/rails.png"

2.3.3 JavaScript/CoffeeScript and ERB

If you add an erb extension to a JavaScript asset, making it something such as
application.js.erb, you can then use the asset_path helper in your
JavaScript code:

$('#logo').attr({ src: "<%= asset_path('logo.png') %>" });

This writes the path to the particular asset being referenced.

Similarly, you can use the asset_path helper in CoffeeScript files with erb
extension (e.g., application.coffee.erb):

$('#logo').attr src: "<%= asset_path('logo.png') %>"

2.4 Manifest Files and Directives

Sprockets uses manifest files to determine which assets to include and serve.
These manifest files contain directives - instructions that tell Sprockets
which files to require in order to build a single CSS or JavaScript file. With
these directives, Sprockets loads the files specified, processes them if
necessary, concatenates them into one single file and then compresses them (if
Rails.application.config.assets.compress is true). By serving one file rather
than many, the load time of pages can be greatly reduced because the browser
makes fewer requests. Compression also reduces file size, enabling the
browser to download them faster.

For example, a new Rails 4 application includes a default
app/assets/javascripts/application.js file containing the following lines:

// ...
//= require jquery
//= require jquery_ujs
//= require_tree .

In JavaScript files, Sprockets directives begin with //=. In the above case,
the file is using the require and the require_tree directives. The require
directive is used to tell Sprockets the files you wish to require. Here, you are
requiring the files jquery.js and jquery_ujs.js that are available somewhere
in the search path for Sprockets. You need not supply the extensions explicitly.
Sprockets assumes you are requiring a .js file when done from within a .js
file.

The require_tree directive tells Sprockets to recursively include all
JavaScript files in the specified directory into the output. These paths must be
specified relative to the manifest file. You can also use the
require_directory directive which includes all JavaScript files only in the
directory specified, without recursion.

Directives are processed top to bottom, but the order in which files are
included by require_tree is unspecified. You should not rely on any particular
order among those. If you need to ensure some particular JavaScript ends up
above some other in the concatenated file, require the prerequisite file first
in the manifest. Note that the family of require directives prevents files
from being included twice in the output.

Rails also creates a default app/assets/stylesheets/application.css file
which contains these lines:

/* ...
*= require_self
*= require_tree .
*/

Rails 4 creates both app/assets/javascripts/application.js and
app/assets/stylesheets/application.css regardless of whether the
--skip-sprockets option is used when creating a new rails application. This is
so you can easily add asset pipelining later if you like.

The directives that work in JavaScript files also work in stylesheets
(though obviously including stylesheets rather than JavaScript files). The
require_tree directive in a CSS manifest works the same way as the JavaScript
one, requiring all stylesheets from the current directory.

In this example, require_self is used. This puts the CSS contained within the
file (if any) at the precise location of the require_self call.

If you want to use multiple Sass files, you should generally use the Sass @import rule
instead of these Sprockets directives. When using Sprockets directives, Sass files exist within
their own scope, making variables or mixins only available within the document they were defined in.

You can do file globbing as well using @import "*", and @import "**/*" to add the whole tree which is equivalent to how require_tree works. Check the sass-rails documentation for more info and important caveats.

You can have as many manifest files as you need. For example, the admin.css
and admin.js manifest could contain the JS and CSS files that are used for the
admin section of an application.

The same remarks about ordering made above apply. In particular, you can specify
individual files and they are compiled in the order specified. For example, you
might concatenate three CSS files together this way:

/* ...
*= require reset
*= require layout
*= require chrome
*/

2.5 Preprocessing

The file extensions used on an asset determine what preprocessing is applied.
When a controller or a scaffold is generated with the default Rails gemset, a
CoffeeScript file and a SCSS file are generated in place of a regular JavaScript
and CSS file. The example used before was a controller called "projects", which
generated an app/assets/javascripts/projects.coffee and an
app/assets/stylesheets/projects.scss file.

In development mode, or if the asset pipeline is disabled, when these files are
requested they are processed by the processors provided by the coffee-script
and sass gems and then sent back to the browser as JavaScript and CSS
respectively. When asset pipelining is enabled, these files are preprocessed and
placed in the public/assets directory for serving by either the Rails app or
web server.

Additional layers of preprocessing can be requested by adding other extensions,
where each extension is processed in a right-to-left manner. These should be
used in the order the processing should be applied. For example, a stylesheet
called app/assets/stylesheets/projects.scss.erb is first processed as ERB,
then SCSS, and finally served as CSS. The same applies to a JavaScript file -
app/assets/javascripts/projects.coffee.erb is processed as ERB, then
CoffeeScript, and served as JavaScript.

Keep in mind the order of these preprocessors is important. For example, if
you called your JavaScript file app/assets/javascripts/projects.erb.coffee
then it would be processed with the CoffeeScript interpreter first, which
wouldn't understand ERB and therefore you would run into problems.

3 In Development

In development mode, assets are served as separate files in the order they are
specified in the manifest file.

3.1 Runtime Error Checking

By default the asset pipeline will check for potential errors in development mode during
runtime. To disable this behavior you can set:

config.assets.raise_runtime_errors = false

When this option is true, the asset pipeline will check if all the assets loaded
in your application are included in the config.assets.precompile list.
If config.assets.digest is also true, the asset pipeline will require that
all requests for assets include digests.

3.2 Turning Digests Off

You can turn off digests by updating config/environments/development.rb to
include:

config.assets.digest = false

When this option is true, digests will be generated for asset URLs.

3.3 Turning Debugging Off

You can turn off debug mode by updating config/environments/development.rb to
include:

config.assets.debug = false

When debug mode is off, Sprockets concatenates and runs the necessary
preprocessors on all files. With debug mode turned off the manifest above would
generate instead:

<script src="/assets/application.js"></script>

Assets are compiled and cached on the first request after the server is started.
Sprockets sets a must-revalidate Cache-Control HTTP header to reduce request
overhead on subsequent requests - on these the browser gets a 304 (Not Modified)
response.

If any of the files in the manifest have changed between requests, the server
responds with a new compiled file.

You can also enable compression in development mode as a sanity check, and
disable it on-demand as required for debugging.

4 In Production

In the production environment Sprockets uses the fingerprinting scheme outlined
above. By default Rails assumes assets have been precompiled and will be
served as static assets by your web server.

During the precompilation phase an MD5 is generated from the contents of the
compiled files, and inserted into the filenames as they are written to disc.
These fingerprinted names are used by the Rails helpers in place of the manifest
name.

Note: with the Asset Pipeline the :cache and :concat options aren't used
anymore, delete these options from the javascript_include_tag and
stylesheet_link_tag.

The fingerprinting behavior is controlled by the config.assets.digest
initialization option (which defaults to true for production and false for
everything else).

Under normal circumstances the default config.assets.digest option
should not be changed. If there are no digests in the filenames, and far-future
headers are set, remote clients will never know to refetch the files when their
content changes.

4.1 Precompiling Assets

Rails comes bundled with a rake task to compile the asset manifests and other
files in the pipeline.

Compiled assets are written to the location specified in config.assets.prefix.
By default, this is the /assets directory.

You can call this task on the server during deployment to create compiled
versions of your assets directly on the server. See the next section for
information on compiling locally.

The rake task is:

$ RAILS_ENV=production bin/rake assets:precompile

Capistrano (v2.15.1 and above) includes a recipe to handle this in deployment.
Add the following line to Capfile:

load 'deploy/assets'

This links the folder specified in config.assets.prefix to shared/assets.
If you already use this shared folder you'll need to write your own deployment
task.

It is important that this folder is shared between deployments so that remotely
cached pages referencing the old compiled assets still work for the life of
the cached page.

The default matcher for compiling files includes application.js,
application.css and all non-JS/CSS files (this will include all image assets
automatically) from app/assets folders including your gems:

The matcher (and other members of the precompile array; see below) is
applied to final compiled file names. This means anything that compiles to
JS/CSS is excluded, as well as raw JS/CSS files; for example, .coffee and
.scss files are not automatically included as they compile to JS/CSS.

If you have other manifests or individual stylesheets and JavaScript files to
include, you can add them to the precompile array in config/initializers/assets.rb:

Always specify an expected compiled filename that ends with .js or .css,
even if you want to add Sass or CoffeeScript files to the precompile array.

The rake task also generates a manifest-md5hash.json that contains a list with
all your assets and their respective fingerprints. This is used by the Rails
helper methods to avoid handing the mapping requests back to Sprockets. A
typical manifest file looks like:

The default location for the manifest is the root of the location specified in
config.assets.prefix ('/assets' by default).

If there are missing precompiled files in production you will get an
Sprockets::Helpers::RailsHelper::AssetPaths::AssetNotPrecompiledError
exception indicating the name of the missing file(s).

4.1.1 Far-future Expires Header

Precompiled assets exist on the file system and are served directly by your web
server. They do not have far-future headers by default, so to get the benefit of
fingerprinting you'll have to update your server configuration to add those
headers.

4.1.2 GZip Compression

When files are precompiled, Sprockets also creates a
gzipped (.gz) version of your assets. Web
servers are typically configured to use a moderate compression ratio as a
compromise, but since precompilation happens once, Sprockets uses the maximum
compression ratio, thus reducing the size of the data transfer to the minimum.
On the other hand, web servers can be configured to serve compressed content
directly from disk, rather than deflating non-compressed files themselves.

This directive is available if the core module that provides this feature was
compiled with the web server. Ubuntu/Debian packages, even nginx-light, have
the module compiled. Otherwise, you may need to perform a manual compilation:

./configure --with-http_gzip_static_module

If you're compiling NGINX with Phusion Passenger you'll need to pass that option
when prompted.

A robust configuration for Apache is possible but tricky; please Google around.
(Or help update this Guide if you have a good configuration example for Apache.)

4.2 Local Precompilation

There are several reasons why you might want to precompile your assets locally.
Among them are:

You may not have write access to your production file system.

You may be deploying to more than one server, and want to avoid
duplication of work.

You may be doing frequent deploys that do not include asset changes.

Local compilation allows you to commit the compiled files into source control,
and deploy as normal.

There are three caveats:

You must not run the Capistrano deployment task that precompiles assets.

You must ensure any necessary compressors or minifiers are
available on your development system.

You must change the following application configuration setting:

In config/environments/development.rb, place the following line:

config.assets.prefix = "/dev-assets"

The prefix change makes Sprockets use a different URL for serving assets in
development mode, and pass all requests to Sprockets. The prefix is still set to
/assets in the production environment. Without this change, the application
would serve the precompiled assets from /assets in development, and you would
not see any local changes until you compile assets again.

In practice, this will allow you to precompile locally, have those files in your
working tree, and commit those files to source control when needed. Development
mode will work as expected.

4.3 Live Compilation

In some circumstances you may wish to use live compilation. In this mode all
requests for assets in the pipeline are handled by Sprockets directly.

To enable this option set:

config.assets.compile = true

On the first request the assets are compiled and cached as outlined in
development above, and the manifest names used in the helpers are altered to
include the MD5 hash.

Sprockets also sets the Cache-Control HTTP header to max-age=31536000. This
signals all caches between your server and the client browser that this content
(the file served) can be cached for 1 year. The effect of this is to reduce the
number of requests for this asset from your server; the asset has a good chance
of being in the local browser cache or some intermediate cache.

This mode uses more memory, performs more poorly than the default and is not
recommended.

If you are deploying a production application to a system without any
pre-existing JavaScript runtimes, you may want to add one to your Gemfile:

group :production do
gem 'therubyracer'
end

4.4 CDNs

CDN stands for Content Delivery
Network, they are
primarily designed to cache assets all over the world so that when a browser
requests the asset, a cached copy will be geographically close to that browser.
If you are serving assets directly from your Rails server in production, the
best practice is to use a CDN in front of your application.

A common pattern for using a CDN is to set your production application as the
"origin" server. This means when a browser requests an asset from the CDN and
there is a cache miss, it will grab the file from your server on the fly and
then cache it. For example if you are running a Rails application on
example.com and have a CDN configured at mycdnsubdomain.fictional-cdn.com,
then when a request is made to mycdnsubdomain.fictional-
cdn.com/assets/smile.png, the CDN will query your server once at
example.com/assets/smile.png and cache the request. The next request to the
CDN that comes in to the same URL will hit the cached copy. When the CDN can
serve an asset directly the request never touches your Rails server. Since the
assets from a CDN are geographically closer to the browser, the request is
faster, and since your server doesn't need to spend time serving assets, it can
focus on serving application code as fast as possible.

4.4.1 Set up a CDN to Serve Static Assets

To set up your CDN you have to have your application running in production on
the internet at a publically available URL, for example example.com. Next
you'll need to sign up for a CDN service from a cloud hosting provider. When you
do this you need to configure the "origin" of the CDN to point back at your
website example.com, check your provider for documentation on configuring the
origin server.

The CDN you provisioned should give you a custom subdomain for your application
such as mycdnsubdomain.fictional-cdn.com (note fictional-cdn.com is not a
valid CDN provider at the time of this writing). Now that you have configured
your CDN server, you need to tell browsers to use your CDN to grab assets
instead of your Rails server directly. You can do this by configuring Rails to
set your CDN as the asset host instead of using a relative path. To set your
asset host in Rails, you need to set config.action_controller.asset_host in
config/production.rb:

You only need to provide the "host", this is the subdomain and root
domain, you do not need to specify a protocol or "scheme" such as http:// or
https://. When a web page is requested, the protocol in the link to your asset
that is generated will match how the webpage is accessed by default.

You can also set this value through an environment
variable to make running a
staging copy of your site easier:

config.action_controller.asset_host = ENV['CDN_HOST']

Note: You would need to set CDN_HOST on your server to mycdnsubdomain
.fictional-cdn.com for this to work.

Once you have configured your server and your CDN when you serve a webpage that
has an asset:

<%= asset_path('smile.png') %>

Instead of returning a path such as /assets/smile.png (digests are left out
for readability). The URL generated will have the full path to your CDN.

http://mycdnsubdomain.fictional-cdn.com/assets/smile.png

If the CDN has a copy of smile.png it will serve it to the browser and your
server doesn't even know it was requested. If the CDN does not have a copy it
will try to find it a the "origin" example.com/assets/smile.png and then store
it for future use.

If you want to serve only some assets from your CDN, you can use custom :host
option your asset helper, which overwrites value set in
config.action_controller.asset_host.

4.4.2 Customize CDN Caching Behavior

A CDN works by caching content. If the CDN has stale or bad content, then it is
hurting rather than helping your application. The purpose of this section is to
describe general caching behavior of most CDNs, your specific provider may
behave slightly differently.

4.4.2.1 CDN Request Caching

While a CDN is described as being good for caching assets, in reality caches the
entire request. This includes the body of the asset as well as any headers. The
most important one being Cache-Control which tells the CDN (and web browsers)
how to cache contents. This means that if someone requests an asset that does
not exist /assets/i-dont-exist.png and your Rails application returns a 404,
then your CDN will likely cache the 404 page if a valid Cache-Control header
is present.

4.4.2.2 CDN Header Debugging

One way to check the headers are cached properly in your CDN is by using curl. You
can request the headers from both your server and your CDN to verify they are
the same:

Check your CDN documentation for any additional information they may provide
such as X-Cache or for any additional headers they may add.

4.4.2.3 CDNs and the Cache-Control Header

The cache control
header is a W3C
specification that describes how a request can be cached. When no CDN is used, a
browser will use this information to cache contents. This is very helpful for
assets that are not modified so that a browser does not need to re-download a
website's CSS or javascript on every request. Generally we want our Rails server
to tell our CDN (and browser) that the asset is "public", that means any cache
can store the request. Also we commonly want to set max-age which is how long
the cache will store the object before invalidating the cache. The max-age
value is set to seconds with a maximum possible value of 31536000 which is one
year. You can do this in your rails application by setting

config.static_cache_control = "public, max-age=31536000"

Now when your application serves an asset in production, the CDN will store the
asset for up to a year. Since most CDNs also cache headers of the request, this
Cache-Control will be passed along to all future browsers seeking this asset,
the browser then knows that it can store this asset for a very long time before
needing to re-request it.

4.4.2.4 CDNs and URL based Cache Invalidation

Most CDNs will cache contents of an asset based on the complete URL. This means
that a request to

http://mycdnsubdomain.fictional-cdn.com/assets/smile-123.png

Will be a completely different cache from

http://mycdnsubdomain.fictional-cdn.com/assets/smile.png

If you want to set far future max-age in your Cache-Control (and you do),
then make sure when you change your assets that your cache is invalidated. For
example when changing the smiley face in an image from yellow to blue, you want
all visitors of your site to get the new blue face. When using a CDN with the
Rails asset pipeline config.assets.digest is set to true by default so that
each asset will have a different file name when it is changed. This way you
don't have to ever manually invalidate any items in your cache. By using a
different unique asset name instead, your users get the latest asset.

5 Customizing the Pipeline

5.1 CSS Compression

One of the options for compressing CSS is YUI. The YUI CSS
compressor provides
minification.

The following line enables YUI compression, and requires the yui-compressor
gem.

config.assets.css_compressor = :yui

The other option for compressing CSS if you have the sass-rails gem installed is

config.assets.css_compressor = :sass

5.2 JavaScript Compression

Possible options for JavaScript compression are :closure, :uglifier and
:yui. These require the use of the closure-compiler, uglifier or
yui-compressor gems, respectively.

The default Gemfile includes uglifier.
This gem wraps UglifyJS (written for
NodeJS) in Ruby. It compresses your code by removing white space and comments,
shortening local variable names, and performing other micro-optimizations such
as changing if and else statements to ternary operators where possible.

The following line invokes uglifier for JavaScript compression.

config.assets.js_compressor = :uglifier

You will need an ExecJS
supported runtime in order to use uglifier. If you are using Mac OS X or
Windows you have a JavaScript runtime installed in your operating system.

The config.assets.compress initialization option is no longer used in
Rails 4 to enable either CSS or JavaScript compression. Setting it will have no
effect on the application. Instead, setting config.assets.css_compressor and
config.assets.js_compressor will control compression of CSS and JavaScript
assets.

5.3 Using Your Own Compressor

The compressor config settings for CSS and JavaScript also take any object.
This object must have a compress method that takes a string as the sole
argument and it must return a string.

To enable this, pass a new object to the config option in application.rb:

config.assets.css_compressor = Transformer.new

5.4 Changing the assets Path

The public path that Sprockets uses by default is /assets.

This can be changed to something else:

config.assets.prefix = "/some_other_path"

This is a handy option if you are updating an older project that didn't use the
asset pipeline and already uses this path or you wish to use this path for
a new resource.

5.5 X-Sendfile Headers

The X-Sendfile header is a directive to the web server to ignore the response
from the application, and instead serve a specified file from disk. This option
is off by default, but can be enabled if your server supports it. When enabled,
this passes responsibility for serving the file to the web server, which is
faster. Have a look at send_file
on how to use this feature.

Apache and NGINX support this option, which can be enabled in
config/environments/production.rb:

If you are upgrading an existing application and intend to use this
option, take care to paste this configuration option only into production.rb
and any other environments you define with production behavior (not
application.rb).

For further details have a look at the docs of your production web server:
- Apache
- NGINX

6 Assets Cache Store

The default Rails cache store will be used by Sprockets to cache assets in
development and production. This can be changed by setting
config.assets.cache_store:

config.assets.cache_store = :memory_store

The options accepted by the assets cache store are the same as the application's
cache store.

7 Adding Assets to Your Gems

Assets can also come from external sources in the form of gems.

A good example of this is the jquery-rails gem which comes with Rails as the
standard JavaScript library gem. This gem contains an engine class which
inherits from Rails::Engine. By doing this, Rails is informed that the
directory for this gem may contain assets and the app/assets, lib/assets and
vendor/assets directories of this engine are added to the search path of
Sprockets.

8 Making Your Library or Gem a Pre-Processor

As Sprockets uses Tilt as a generic
interface to different templating engines, your gem should just implement the
Tilt template protocol. Normally, you would subclass Tilt::Template and
reimplement the prepare method, which initializes your template, and the
evaluate method, which returns the processed source. The original source is
stored in data. Have a look at
Tilt::Template
sources to learn more.

module BangBang
class Template < ::Tilt::Template
def prepare
# Do any initialization here
end
# Adds a "!" to original template.
def evaluate(scope, locals, &block)
"#{data}!"
end
end
end

Now that you have a Template class, it's time to associate it with an
extension for template files:

Sprockets.register_engine '.bang', BangBang::Template

9 Upgrading from Old Versions of Rails

There are a few issues when upgrading from Rails 3.0 or Rails 2.x. The first is
moving the files from public/ to the new locations. See Asset
Organization above for guidance on the correct locations
for different file types.

Next will be avoiding duplicate JavaScript files. Since jQuery is the default
JavaScript library from Rails 3.1 onwards, you don't need to copy jquery.js
into app/assets and it will be included automatically.

The third is updating the various environment files with the correct default
options.

In application.rb:

# Version of your assets, change this if you want to expire all your assets
config.assets.version = '1.0'
# Change the path that assets are served from config.assets.prefix = "/assets"

Feedback

You may also find incomplete content, or stuff that is not up to date.
Please do add any missing documentation for master. Make sure to check
Edge Guides first to verify
if the issues are already fixed or not on the master branch.
Check the Ruby on Rails Guides Guidelines
for style and conventions.

If for whatever reason you spot something to fix but cannot patch it yourself, please
open an issue.